A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
A time-of-flight mass spectrometer (TOF MS) utilizes a high-resolution mass analyzer (TOF analyzer). Ions are transported from the ion source into the TOF entrance region through a series of ion guides and ion lenses. The TOF analyzer extracts ions in pulses (or packets) into an electric field-free drift tube. In the drift tube, ions of differing masses travel at different velocities and thus separate (spread out) according to their differing masses, enabling mass resolution based on time-of-flight.
To elucidate additional information regarding a sample, an MS system may be configured for carrying out tandem MS, or MS-MS, experiments. MS-MS may be implemented in a TOF-based system, an example of which is the qTOF system. The qTOF system utilizes, in series, a first quadrupole device, a second quadrupole device, and a TOF analyzer. The first quadrupole device is operated as a mass filter to select a single ion mass or mass range from the larger range of ions provided by the ion source. The second quadrupole device is typically an RF-only device enclosed in a gas chamber and utilized as a collision cell to dissociate the selected ions, or “parent” ions, into fragment ions (or “daughter” ions) by collision induced dissociation (CID). Parent ions and/or fragment ions may be analyzed by the TOF analyzer. If parent ion spectra are desired at the high resolution obtained in the TOF section, the gas in the collision cell is often left at its normal pressure but the ions entering the collision cell are reduced to a low enough energy to avoid fragmentation. In all these arrangements, typically there is no means available for limiting the ion signal in cases where the detector system, e.g. the electronics employed in collecting/accumulating and processing ion signals, is saturated or overloaded.
Therefore, there is a need for TOF MS systems and methods that provide a way to limit the number of ions entering the TOF analyzer to avoid detector system saturation.